The present invention relates to an improvement in a hydraulic reaction device which can be suitably used in a power steering apparatus to correctly control a steering force in accordance with various vehicle driving conditions such as a vehicle speed.
Various types of hydraulic reaction device of vehicle speed responsive type for performing steering force control corresponding to a running speed of a vehicle (vehicle speed) in a power steering apparatus for reducing a steering operation force (steering force) of an automobile have been proposed. That is, when a vehicle is stopped or runs at a low speed, a reaction oil pressure is minimized to enable light steering operation. When a vehicle runs at a high speed, the reaction oil pressure is increased to realize a heavy steering wheel operation feeling, thereby obtaining stability in straight running. This steering force control is performed by pivoting or constraining input and output shafts of the power steering apparatus relative to each other by a reaction piston for selectively constraining the input and output shafts in accordance with the magnitude of the reaction oil pressure.
As conventional hydraulic reaction devices of this type, Japanese Patent Laid-Open Nos. 61-105273 and 61-132466, Japanese Utility Model Laid-Open No. 62-25265, and Japanese Patent Laid-Open No. 63-68467 propose arrangements in which a reaction oil pressure is divided from a portion of a main oil pressure path connected from a pump to a power cylinder via a flow path switching valve and is controlled by a reaction oil pressure control valve constituted by, e.g., a spool valve so as to be supplied to a hydraulic reaction chamber for moving a reaction piston. In each of these conventional devices, as a driving source of the reaction oil pressure control valve described above, an electric actuator such as a solenoid coil or a stepping motor capable of generating a predetermined operation force by an output current from a controller in accordance with a detection signal from a vehicle speed sensor, a steering angle sensor, or a torque sensor is generally used. The hydraulic reaction device is correctly and reliably operated by such electronic control to perform steering force control corresponding to a vehicle speed or steering conditions.
According to the above conventional devices, however, the reaction oil pressure control valve is required to control the reaction oil pressure supplied to the hydraulic reaction chamber in accordance with vehicle speed or the like, and the actuator such as a solenoid coil driven by a controller using a microcomputer or the like having a converter for converting a current output in accordance with a detection signal from, e.g., a vehicle speed sensor is required as a driving source for driving the control valve. As a result, the number of components is increased to complicate the structure of the device, thereby increasing a manufacturing cost. In addition, in the above controller, a countermeasure against a disturbance such as an electromagnetic wave interference is essential to perform stable control. As a result, the manufacturing cost is further increased.
For this reason, Japanese Utility Model Publication No. 61-32851 proposes a simple hydraulic reaction device in which a vehicle-speed responsive pump capable of increasing/decreasing its discharge flow rate in accordance with an increase/decrease in vehicle speed is used independently of a main pump as a reaction oil pressure source. This device has a great advantage of a simpler arrangement than those of the above conventional devices. In such a simple device, however, since the vehicle-speed responsive pump cannot be directly connected to the hydraulic reaction chamber, an oil pressure-controlled spool valve having a spool operated in accordance with the discharge flow rate from the pump to control the reaction oil pressure to the hydraulic reaction chamber in a predetermined state must be used as a pressure control valve. In particular, the above pressure control valve increases the discharge flow rate from the pump in accordance with a vehicle speed to drive the spool by using a pressure difference produced between the upstream and downstream sides of variable and fixed throttle portions arranged in a path of the spool and supplies the reaction oil pressure to the hydraulic reaction chamber through a reaction path extending from the upstream side of the two throttle portions. Therefore, the structure and control of the valve are complicated to increase manufacturing cost.
In addition, even if the above pressure control valve is used, since a subpump hydraulic system constituted by the vehicle-speed responsive pump is arranged to control a reaction oil pressure independently of a main pump hydraulic system of the power steering apparatus, the pressure control valve responds to only a vehicle speed but is not related to other various driving conditions of a vehicle, e.g., a steering state such as an input torque to a steering wheel or an operation in the main pump hydraulic system such as external conditions, e.g., kick-back from a road. Therefore, steering reaction control in which steering force control can be performed by a power cylinder output in correspondence with various driving conditions cannot be expected. For example, a force required to operate a steering wheel is desired to be increased in proportion to an increase in vehicle speed and an increase in power cylinder output, i.e., steering resistance. However, the subpump hydraulic system which can obtain only a reaction oil pressure controlled simply by a vehicle speed cannot satisfy these requirements. Therefore, a demand has arisen for a certain countermeasure capable of solving the above problems.
In particular, in a hydraulic reaction device of this type, an arrangement must be simplified to reduce manufacturing cost, and the reaction oil pressure must be proportionally controlled to have a magnitude or characteristics satisfactory to produce a steering reaction force capable of obtaining a required steering force in accordance with a vehicle speed or driving conditions, thereby obtaining a correct and reliable steering reaction force to perform steering force control. Therefore, these requirements must be taken into consideration.